Longitudinal implant

ABSTRACT

The longitudinal implant is fastened to bones on either side of a damaged area through a connecting device. Said implant is comprised of a filament or fiber composite material and said connecting device is made of a material harder than said longitudinal implant. The longitudinal implant is preferably made of a carbon filament composite material, wherein the filament are encapsulated in a polymer matrix.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation of U.S. application Ser. No. 10/506,803 filedSep. 7, 2004,which is a §371 National Stage Application ofPCT/CH02/00136 filed Mar. 6, 2002; the entire disclosure of the priorapplications are considered part of the disclosure of the accompanyingContinuation Application and are hereby incorporated by reference.

This invention relates to a longitudinal implant and connecting devicewherein said longitudinal implant is fastened to bones on either side ofa damaged area through said connecting device.

In some spinal repair situations, the damaged area of the spine isspanned by a slotted plate through which pedicle screws are inserted andfastened to the pedicle bones on either side of a damaged area. Thisfixes the spatial distance between the pedicle bones and therefore fixesthe distance between vertebrae so that the damaged area of the spine canbe repaired. In other spinal situations, the damaged area of the spineis spanned by a rod. At least two connectors are slidable along the rodconnecting pedicle screws or hooks to the rod. Such a rod and fixationsystem is disclosed in EP 0 923 908A (Robert Lange).

Spinal repair is often times accomplished with hollow cages in whichbone fragments are inserted that will grow to an extent to fuse theupper and lower vertebrae together at the damaged area. By fixing andholding the distance between these vertebrae, the bone in the cages willhave time to grow and join the vertebrae together.

It is an objective of this invention to provide an elongated implant andpedicle screw or hook fixation system providing an increased stability.

The implant of the device according to this invention is comprised of afilament composite material—and said pedicle screws or hooks are made ofa material harder than said implant. The implant provides a surface thathas more friction than a titanium implant. If the implant is a platehaving a longitudinal slot, the plate is placed between a nut and anupper surface of a pedicle screw. The plate can be squeezed and lockedinto position because of the squeezing and the increased frictionbetween titanium and the filament composite material. When all membersare titanium, the required position is not always available andindentations are often provided along the slot.

Fixation systems manufactured from metals such as titanium alloy andstainless steel confound postoperative radiologic assessments becausethey are radiopac and can produce artifact. The use of an implantcomprised of a fiber reinforced polymer composite permits betterdiagnostic assessment of soft tissue and bone by normal radiographicmethods.

According to a preferred embodiment of the invention, the fibers arealigned lengthwise, so that compression will not change their strengthcharacteristics to any extent even when compressed. Preferably thefibers or filaments are oriented to resist biomechanical forces.

Other advantages and features of the present invention will be apparentto those skilled in this art reaching the following specification withreference to the accompanying drawings in which:

FIG. 1 is a view of an embodiment of an implant of this invention;

FIG. 2 is a section along line I-I of FIG. 1;

FIG. 3 is a section through an embodiment of the implant with adifferent curvature;

FIG. 4 shows the implant according to FIG. 3 connecting two pediclescrews;

FIG. 5 a schematic view of a material block with horizontal fiber and animplant machined from this block;

FIG. 6 a schematic view of a material block with a curvature fiberorientation and an implant machined from this block;

FIG. 7 a side view of a connecting device of this invention;

FIG. 8 a partial section through the connecting device;

FIG. 9 a side view of an implant testing configuration;

FIGS. 10 and 11 overall views showing rods or rails connecting twovertebrae of a spinal cord;

FIG. 12 a perspective view of another embodiment of an implant of thisinvention and

FIG. 13 a section along line XIII-XIII of FIG. 12.

FIGS. 1 and 2 disclose a plate 1 having a longitudinal slot 2 extendingalong a substantial portion of its length. The plate has a curvature ofabout 100 as shown in FIG. 2. FIG. 3 discloses a plate 1′ which has acurvature of 20°.

The plates 1 and 1′ as well as rods 30 and rails 17 are manufacturedfrom a composite material composed of long filaments or fibers 18 and 19encapsulated in a matrix 4 as shown in FIG. 13. The filaments or fibers18 and 19 are preferably long carbon filaments and the matrix ispreferably a polymer. Preferably the carbon filaments or fibers 18 and19 are encapsulated in the polymer polyether-ketoneetherketoneketone(PEMKK). PEKEKK is a known biocompatible polymer. Another possiblepolymer is polyetheretherketone (PEEK). PFKEKK is preferred to PEEKbecause of its greater physical and chemical resistance properties.These characteristics impart greater stability to the plates 1 and 1′,rods 30 and rails 17 or other connecting parts during a long-termimplantation.

FIGS. 4, 7 and 8 disclose a pedical plate fixation systems 5 and 5′comprising a plate 1′, two bone screws 6 and two nuts 7. The screws 6and the nuts 7 are manufactured from steel or medical grade titaniumalloy. Bone screws 6 are common in the orthopedic arts. The screws 6 areprovided with bone engaging threads 6 a and at its other end a screwsegment 6 b with a conventional thread. The thread 6 a flairs outwardlyto an enlarged portion 6 c. The enlarged portions 6 c have a widthgreater than the width of the slot 2. The screw segment 6 b extendsoutwardly from the enlarged portion 6 c and extends through the slot 2.The nut 7 is received by the screw segment 6 b,and since the slot 2 ismanufactured without any additional recesses or indentations, bevels orslants (see FIGS. 1, 2, 3, 4, 7 and 8) the plate 1′ can be graspedbetween the enlarged portion 6 c and the nut 7 to tightly secure theplate 1 by threading the nut 7 toward the enlarged portion 6 c.

The embodiment according to FIG. 7 is provided with slip washers 20, 21and 22 having a planar surface 20 a,21 a,22 a and a concave or convexsurface 20 b,21 b,22 b. The planar surfaces 20 a and 21 a are touchingthe plate 1′ are preferably provided with rips (not shown), which aredepressed in the plate 1′ and which prevent the screw 6 from movingalong the slot 2.

As the carbon-filament composition material of the plate 1 is softerthan titanium and at its surface is somewhat rougher than a titaniumsurface, the plate 1 can be squeezed between the enlarged portion 6 cand the nut 7. This prevents the screws 6 from moving along the slot 2both by depression caused by the squeezing and the enhanced frictionthere between.

The filaments 3 encapsulated in the polymer matrix 4 are oriented asshown in FIGS. 7 and B. The plate 1 is machined from a block 8 having acurvature fiber orientation as indicated in FIG. 6. The curvature of thefiber lay-up is the same as the curvature of the plate 1 or plate 2. Thefilaments 3 in the matrix 4 are therefore aligned lengthwise of the slot2. The FIG. 5 shows a block 8′ with a parallel curvature lay-up and thein the plate 1′ machined from this block 8′ the filaments do not followthe curvature of the plate 11 and are shorter. With the testingconfiguration 10 disclosed in FIG. 9, tests were conducted using theASTM provisional standard for spinal implants. This mechanical testinghas shown that the strength of the plate 1 and 1′ is similar to platesmade from stainless steel and titanium. The goal of stabilization hasnot been sacrificed. As material of the plates 1 and 1′ is radiolucent,the plates do not interfere with diagnostic methods.

As already mentioned, the implant according to this invention can alsobe a rod 12 as disclosed in FIGS. 10 and 11 ox a rail according to FIGS.12 and 13. Two connectors 13 include two clamping members 14 connectingpedicle screws 15 to said rods 12. The rods 12 and rails are made fromthe same material as the plates 1 and 1′ and the filaments encapsulatedin the matrix are preferably oriented in an axial direction. A rail 17with a rectangular cross section as shown in FIGS. 12 and 13 is morestable to rotation than a rod.

Another advantage of implants manufactured from a carbon filamentcomposite material is that its strength, flexibility and hardness can bevaried by changing the ratio of filaments to plastic. It has been found,that “bone growth” is enhanced when it is under a certain degree ofphysiological stress. Thus, it will be desirable to select a compositeratio for the plate to gain the required degree of stiffness withoutsacrificing any strength. The ratio of filaments to plastic ispreferably higher than 40% (weight) and more preferably higher than 60%(weight).

The filaments of fibers are not randomly embedded, but oriented inlayers A as shown in FIG. A. The layers A can be parallel to each otherand to a surface 23 as shown in FIG. 13. The layers A may be made up ofwoven filaments 18 and 19. The filaments 18 are oriented in the axialdirection of the longitudinal implant 1, 17 and 30. The filaments 19 areoriented perpendicular to the axial direction. The filaments 18 and 19are oriented to resist the biomechanical forces as for example bendingforce as shown in FIG. 9. The filaments can also be coiled to resisttorsion forces. The distance D between two layers A is preferably lessthan 0.5 mm and preferably about 0.1 mm.

1. A longitudinal implant and connecting device wherein saidlongitudinal implant is fastenable to bones on either side of a damagedarea through said connecting device, said implant and connecting devicecomprising: a longitudinal implant made of a filament or fiber compositematerial, wherein filaments or fibers in said material are oriented toresist biomechanical forces, and a connecting device made of a materialharder than said longitudinal implant, wherein, said connecting deviceis operative to squeeze and lock the longitudinal implant into positionboth by depression caused by a squeezing and increased friction betweenthe harder material of the connecting device and the composite materialof the longitudinal implant, and wherein the implant is one of anelongated structure, the structure having a longitudinal slot extendingalong a substantial portion of its length and wherein filaments orfibers are aligned lengthwise, so that compression will not change theirstrength characteristics to any extent even when compressed, and whereinthe implant is formed without any recesses, bevels, or slants, adjacentto the longitudinal slot.
 2. The longitudinal implant and connectingdevice according to claim 1, wherein the longitudinal implant is made ofa carbon filament composite material.
 3. The longitudinal implant andconnecting device according to claim 1 or 2, wherein the filaments areencapsulated in a polymer matrix.
 4. The longitudinal implant andconnecting device according to claim 3, wherein the filaments areencapsulated in PEEK or PEKEKK.
 5. The longitudinal implant andconnecting device according to claim 1, wherein the connecting devicecomprising a pedicle screw having an upper section having a widthgreater than the width of said slot and an exteriorly threaded portionextending outwardly from said section and extending through said slot.6. The longitudinal implant and connecting device according to claim 5,wherein an interiorally threaded nut is received by an outer end of saidthreaded portion whereby said implant can be grasped between said uppersection and said nut to tightly secure said plate by threading saidupper section.
 7. The longitudinal implant and connecting deviceaccording to claim 1, wherein said connecting device comprises a screwand a nut, each of which is made of titanium.
 8. The longitudinalimplant and connecting device according to claim 1, wherein said implantis a plate.
 9. The longitudinal implant and connecting device accordingto claim 1 or 2, wherein the filaments are woven, comprising firstfilaments that are oriented in the axial direction of the implant, andsecond filaments that are oriented perpendicular to the axial directionof the implant.